1. Field of the Invention
The present invention relates to a casing suitably used, for example, for an in-vehicle electronic control unit in which a circuit board is stored, and therefore, for which a waterproof property is required, and to an assembling method of such a casing.
2. Background Art
There is an electronic control unit including a circuit board on which heating components are mounted and which is hermetically stored in a casing formed of a case and a cover. Heat generated in the heating components is transmitted to the case by contact heat transfer via a heat-transfer material. Also, the case is provided with a connector housing for external connection.
For example, FIG. 2 and FIG. 10 of Patent Document 1 specified below show an electronic control unit that includes a case 2 having an opened ceiling surface and storing therein a circuit board 1, a cover 3 covering the case opening and attached to the case 2, a groove portion (seal groove) 22 provided to either one of the case 2 and the cover 3, a protruding portion 31 provided to the other one of the case 2 and the cover 3 and inserted into the groove portion 22. The cover 3 is attached to the case 2 by inserting the protruding portion 31 into the groove portion 22 via a seal material 30. The both side surfaces of the protruding portion 31 are inclined so that a distance between the side surfaces of the protruding portion 31 and an opposing internal surface of the groove portion 22 increases from top to bottom of the groove portion 22 in a state where the cover 3 is attached to the case 2.
By inclining the side surfaces of the protruding portion 31 inwardly with respect to the internal surface of the groove portion 22 from top to bottom of the groove portion 22, even in a case where a highly viscous adhesive 30 is used as the seal material, it becomes possible to insert the protruding portion 31 into the groove portion 22 by pressing the adhesive 30 with the side surfaces of the protruding portion 31. Hence, a pressure applied to the adhesive 30 can be increased and the adhesive 30 can be spread sufficiently across the side surfaces of the protruding portion 31. Accordingly, because wettability of the adhesive 30 on the side surfaces of the protruding portion 31 is enhanced, an adhesion force is increased. Hence, sealing performance between the case 2 and the cover 3 can be enhanced. In addition, storing portions 26 that store the seal material 30 overflown from the seal groove 22 are provided between a fixing portion 20 and the seal groove 22 of the case 2.
Also, FIG. 1 and FIG. 2 of Patent Document 2 specified below show a bonding structure of a resin case. According to this bonding structure, a convex portion (protruding strip) 7 provided to a member 3 as one of subjects to be bonded and a resin adhesive 5 are placed in a concave portion (groove) 6 provided to a member 2 as the other one of the subjects to be bonded so that the both members 2 and 3 are bonded to each other by allowing the resin adhesive 5 to set. An interval between a side wall 6a of the concave portion 6 and a side wall 7a of the convex portion 7 is set larger in an opening of the concave portion 6 than in an internal portion of the concave portion 6 and the concave portion 6 is provided with a guide portion that guides the convex portion 7 to a predetermined position in the concave portion 6. By setting the interval between the side wall 6a of the concave portion 6 and the side wall 7a of the convex portion 7 larger in the opening of the concave portion 6 than in the internal portion of the concave portion 6, it becomes possible to obtain a satisfactory bonding state by preventing the resin adhesive 5 from being forced out of the concave portion 6 by a capillary phenomenon. Further, it becomes possible to prevent positional displacement between the convex portion 7 and the concave portion 6 in a reliable manner and adverse influences of the capillary phenomenon can be reduced further. In addition, FIG. 7 of Patent Document 2 shows that a step-like adhesive inflow chamber 16 is provided in the opening of the concave portion 6 to prevent the resin adhesive 5 from being forced out from the opening.    Patent Document 1: Japanese Patent No. 4892527 (paragraphs 0032 to 0035, FIG. 2 and FIG. 10)    Patent Document 2: Japanese Patent No. 3355761 (paragraphs 0016, 0017, 0030, and 0031, FIG. 1, FIG. 2, and FIG. 7)
According to the technique of Patent Document 1, the ring-like protruding strip (protruding portion) 31 provided along an outer circumferential portion of the cover 3 is fit into the seal groove 22 provided along an outline outer circumference of the case 2 and a fit-in clearance is tapered so that the fit clearance becomes wide at the bottom of the groove and narrow in the opening of the seal groove 22. However, by taking a variance of dimensions of the respective portions into account, even when positioning pieces 20a and 20b for the cover 3 are provided, it is difficult to reduce a clearance between the seal groove 22 and the base of the ring-like protruding strip 31 to zero in the opening of the seal groove 22 and it is necessary to secure a minimum clearance in consideration of dimensional accuracy of the respective portions. A relative fluctuation rate of a variance of the clearance dimension with respect to a groove width becomes a problem particularly with a large case having a long opposite side distance. When the clearance dimension becomes zero, a thickness of the seal material 30 is reduced to zero partially. This poses a problem that not only waterproof performance is deteriorated, but also an outer shape dimension of the case 2 is increased because of a need to increase a size of the storing portions 26 used to store a surplus of the seal material 30 flowing out to the external surface of the case 2. In addition, fixing screws 28 fixing the case 2 and the cover 3 are provided at positions on an outer side of the seal groove 22. Hence, water does not penetrate into the case 2 where fastened by the fixing screws 28. However, in a case where the cover 3 is made of inexpensive sheet metal, there may be a problem that it becomes difficult to provide a fastening plane portion on the outer side of the protruding portion 31.
According to the technique of Patent Document 2, there is a tapered portion where a width of the opening of the seal groove 6 widens from bottom to top (the tapering direction is opposite to the tapering direction in Patent Document 1) and the protruding strip (convex portion) 7 fit into the seal groove (concave portion) 6 is guided to a fit-in position at a center of the seal groove 6. However, by taking variances of dimensions of the respective portions into account, even when the protruding strip 7 is guided to the fit-in position by the tapered portion, it is difficult to reduce a clearance between the tip end of the protruding strip 7 and the back end of the seal groove 6 to zero. Accordingly, it is necessary to secure a minimum clearance in consideration of dimensional accuracy of the respective portions. A relative fluctuation rate of a variance of the clearance dimension with respect to the groove width becomes a problem particularly with a large case having a long opposite side distance. When the clearance dimension becomes zero, a thickness of the seal material (adhesive) 5 is reduced to zero partially. This poses a problem that not only waterproof performance is deteriorated, but also an outer shape dimension of the case is increased because of a need to increase a size of the inflow chamber 16 used to store a surplus of the seal material 5 flowing out to the external surface of the case.